@Twilightstarbright it's so hard to say over the internet with 0 information about you child.
You may have heard the phrase 'if you've met one person with autism... you've met one person with autism' and that sums it up. All children with autism are so different, they present differently. It's such a wide spectrum, I would never be able to say.
General things could be:
Delay in speech and language development
Sensory issues, things like them being 'funny' with food textures and tastes. Not liking the feeling of certain things, wet, gooey, dry, sandy etc.
Lack of social communication, so lack of eye contact, not responding to their own name, not responding when spoken to, not being able to have a 'normal' back and forth conversation, maybe acting in appropriately or not getting or understanding normal social cues.
Maybe being hypo sensitive to things and climbing repetitively, jumping, spinning to try to feel more of the sensation. Also, flapping hands, tapping body parts, clapping, any repetitive movements. Any way that they can seek a sensory experience, tipping toys out of baskets to make a big crash, repetitively throwing things to bang on the floor etc.
Any other repetitive behaviours, lining things up (this is also typical child development, but is it more than that? Do they line things up ALL the time, do they get upset when you try to stop them?) Do they have a big obsession with doing one specific thing all the time?
Or they could be the complete opposite and be hyper sensitive to things, they may be completely overwhelmed by day to day life and want to be in an enclosed space, think in a den, under a table etc.
This is a VERY general description of things that may concern me in the early years, there are 1000 more things I could type. But also, it's not just how they are on paper, I can see it within 5 minutes of meeting a child, with experience you just 'know.' I think to sum it up, it would be to describe a child as 'being in their own little world' which you hear all the time, but this is not all children with autism, just some.
Good luck to both of you, it can be a tricky process. I hope your children get the support, if they need it.